In rotary machines such as turbines, seals are provided between rotating and stationary components. For example, in steam turbines it is customary to provide a plurality of arcuate packing ring segments to form an annular labyrinth seal between the stationary and rotating components. Typically, the arcuate packing ring segments (typically, four to six per annular seal) are disposed in an annular groove in the stationary component concentric to the axis of rotation of the machine and hence concentric to the sealing surface of the rotating component. Each arcuate seal segment carries an arcuate seal face in opposition to the sealing surface of the rotating component. In labyrinth type seals, the seal faces carry a radially directed array of axially spaced teeth, and which teeth are radially spaced from an array of axially spaced annular teeth forming the sealing surface of the rotating component. The sealing function is achieved by creating turbulent flow of a working media, for example, steam, as it passes through the relatively tight clearances within the labyrinth defined by the seal face teeth and the opposing surface of the rotating component.
The ability to maintain proper clearances without physical contact between the rotating equipment and stationary components allows for the formation of an effective seal. If this radial clearance between the seal faces of the segments and the opposing seal surfaces of the rotating component becomes too large, less turbulence is produced and the sealing action is compromised. Conversely, if the clearance is too tight, the sealing teeth may contact the rotating element, with the result that the teeth lose their sharp profile and tight clearance and thereafter create less turbulence, likewise compromising the sealing action.
In order to create and maintain a desired seal and to avoid damage to the rotor and packing ring during transient conditions, positive pressure, variable clearance packing rings may be used as further disclosed, and hereby incorporated by reference, in: GE Docket No. 193439, Cantor Colburn LLP Docket No. GS1-0202, entitled “Variable Clearance Packing Ring Arrangement”; GE Docket No. 194777, Cantor Colburn LLP Docket No. GS1-0210, entitled “Apparatus and Method for Steam Turbine Variable Clearance Packing”; and GE Docket No. 193440, Cantor Colburn LLP Docket No. GS1-0204, entitled “Variable Clearance Positive Pressure Packing Ring and Carrier Arrangement”; (U.S. Serial Nos. not yet available). In these type of positive pressure, variable clearance packing rings, the packing ring segments are typically spring biased into outer or large clearance positions causing the seal faces carried by the packing ring to be spaced substantially outwardly of the rotary component. After start-up, the working fluid medium, e.g., steam, is inlet to the stationary component, creating a pressure differential which urges the segments to move inwardly against the bias of the springs, toward the inner or small clearance positions. These springs and corresponding ring components are typically located within the annular groove defined by the stationary housing.
However, installation of positive pressure, variable clearance packing rings in existing steam turbines can be a complicated matter which requires field machining or other modification of the rings or of the casing used to mount the rings within the annular groove of the stationary housing. Also, due to circumferential movement of the independent arch segments, retrofitted variable clearance packing rings are prone to archbinding, a condition where an arch segment rides circumferentially over an adjacent segment, jamming the segments in a radially offset position.
Thus, there is a need for a variable clearance positive pressure packing ring which may be easily and simply installed in an annular groove of a stationary component of an existing steam turbine in such manner as to avoid undesired archbinding conditions and excessive extra re-machining of stationary components where retrofit is involved.